Vibrating rate gyro with slaving of detection frequency to excitation frequency

ABSTRACT

The invention relates to a gyroscope comprising at least one mass capable of vibrating along an x axis at a resonant excitation frequency F x  capable of vibrating along a y axis perpendicular to the x axis, at a resonant detection frequency F y , under the effect of a Coriolis force generated by a rotation about a z axis perpendicular to the x and y axes. It includes, connected to the mass or masses, a feedback control loop for controlling the resonant frequency F y  so that F y  is equal or practically equal to F x  throughout the duration of use of the gyroscope.

The invention relates to a vibrating gyroscope.

The operating principle of a vibrating gyroscope is explained in relation to FIG. 1.

A mass M is suspended from a rigid frame C by means of two springs, of stiffness K_(x) and K_(y), It therefore possesses two degrees of freedom, along the x and y directions.

The system may be considered as an assembly of two resonators having eigenfrequencies or natural frequencies F_(x) along x and F_(y) along y.

The mass M is excited at its natural frequency F_(x) along the x axis.

When a speed of rotation Ω about the third, z axis is present, the Coriolis forces induce coupling between the two resonators, causing the mass to vibrate along the y axis.

The amplitude of the movement along y is then proportional to the speed of rotation Ω.

This amplitude is also a function of the difference in the natural frequencies F_(x) and F_(y)—maximum sensitivity is achieved when the two natural frequencies are equal.

In particular, for high-performance gyroscopes, it is necessary to obtain maximum sensitivity of the displacement relative to the speed of rotation. It is therefore very desirable to make these frequencies equal.

However, when the frequency equality condition is met, the bandwidth of the gyroscope becomes very small. To increase it, the detection movement along y is feedback controlled, by applying an electrostatic or electromagnetic force along the y axis to the mass, which force counterbalances the force created by the Coriolis coupling. There is no longer any vibration of the mass along y and it is then the feedback force proportional to the speed of rotation Ω that is measured.

It is therefore desirable in vibrating gyroscopes of higher performance for the movement along the y axis to be feedback controlled and for the frequencies F_(x) and F_(y) to be made coincident.

However, the dispersion due to the method of production in manufacture does not allow a perfectly zero frequency difference to be obtained. It is therefore necessary to make an adjustment in order for the two frequencies to be equal.

A first method consists in making these frequencies equal by mechanical balancing. This therefore involves modifying the mass or stiffness characteristics of one or other of the resonators by removing material. This method may be used for carrying out a coarse initial adjustment of the frequencies.

Another method consists in carrying out electrical balancing. By means of electrodes, a variable electrostatic (or electromagnetic) stiffness is added to one of the two resonators so as to vary its natural frequency. This method allows a very fine initial adjustment of the frequencies to be made using an electrical voltage applied to the electrodes.

If a gyroscope whose frequencies have been initially adjusted by one of these methods is used, the initial adjustment of making the mechanical resonant frequencies F_(x) and F_(y) coincide cannot be maintained in the long term and under all environmental conditions.

This is because parasitic mechanical effects and the thermoelasticity effects are not strictly identical in both resonators and these effects may result in a frequency differentiation when the environmental, both mechanical and thermal, conditions vary.

One important object of the invention is therefore to propose a vibrating gyroscope that allows the initial adjustment of making the mechanical resonant frequencies F_(x) and F_(y) coincident able to be maintained in the long term and under all environmental conditions.

To achieve this object, the invention proposes a gyroscope comprising at least one mass M capable of vibrating along an x axis at a resonant excitation frequency F_(x) and capable of vibrating along a y axis perpendicular to the x axis, at a resonant detection frequency F_(y), under the effect of a Coriolis force generated by a rotation about a z axis perpendicular to the x and y axes, mainly characterized in that it comprises, connected to the mass or masses M, a feedback control loop for controlling the resonant frequency F_(y) so that F_(y) is equal or practically equal to F_(x) throughout the duration of use of the gyroscope.

This feedback control loop thus makes it possible for the stiffness K_(y) to be permanently feedback-controlled so as to make the natural frequencies F_(x) and F_(y) along the two directions equal.

According to one feature of the invention, the gyroscope includes a signal generator for generating a signal that disturbs the vibration of the mass M along y, said generator being connected to the mass M, and the feedback control loop comprises: means for modifying the resonant detection frequency F_(y), means for detecting the variation, induced by the disturbing signal, in the vibration of the mass M along y, an error signal representative of the difference between F_(x) and F_(y) being deduced from this variation, and control means for controlling the F_(y)-modifying means, the control being established on the basis of the error signal.

According to a first embodiment of the invention, the disturbing-signal generator is connected to the mass M via the F_(y)-modifying means.

According to another embodiment, when the gyroscope includes excitation means for exciting the mass M along y with the aim of counterbalancing the vibration along y generated by the Coriolis force, the disturbing-signal generator is connected to the mass M via these excitation means.

Other features and advantages of the invention will become apparent on reading the following detailed description, given by way of nonlimiting example and with reference to the appended drawings in which:

FIG. 1 illustrates schematically the operating principle of a vibrating gyroscope;

FIG. 2 shows schematically the necessary main components relating to a single mass of a gyroscope according to the prior art;

FIG. 3 shows schematically a curve representative of the variation of the amplitude (in dB) of the detection signal |U_(det,y)|, corresponding to the movement of the mass along y, as a function of the frequency in Hz of the excitation signal U_(ex,y) according to the prior art;

FIGS. 4 a) and b) show schematically the curves representative of the control signal (in this case a voltage) for controlling the frequency modulation (FIG. 4 a) and of the perturbing signal U_(ex,y) frequency-modulated about the central frequency F_(x) at the frequency F₀ (FIG. 4 b), expressed as a function of time;

FIGS. 5 a), 5 b) and 5 c) show schematically, according to whether F_(y)>F_(x), F_(y)=F_(x) or F_(y)<F_(x), the curves corresponding to those of FIGS. 3 and 4 a) and also the corresponding variation of the amplitude of the detection signal Δ|U_(det,y)|;

FIG. 6 a) shows schematically the detection signal U_(det,y), the envelope of which is given by Δ|U_(det,y)| for the case in which F_(x)≠F_(y); shown respectively in FIGS. 6 b) and 6 c) are a reference demodulation signal of frequency F₀ and an error signal e;

FIG. 7 shows schematically the necessary main components relating to a signal mass in an example of a gyroscope according to the invention; and

FIG. 8 shows schematically the necessary main components relating to a signal mass of another example of a gyroscope according to the invention.

High-precision vibrating gyroscopes generally have two symmetrical vibrating masses operating in what is called tuning-fork mode.

In micromachined sensors, the excitation movement is generally provided by electrostatic forces along the x direction. These forces are often created by means of electrostatic combs.

The detection movement is picked up along a y direction perpendicular to x. In the case of micromachined sensors produced in a plane structure, this y direction may, depending on the case, lie in the plane of the plane structure or perpendicular to this plane.

FIG. 2 shows the necessary main components relating to a single mass, for the sake of simplicity.

Conventionally, means are provided:

for applying excitation forces along the x direction and for detecting the movement of the masses along x so as to feedback control these excitation forces;

for detecting the movement of the masses along the y direction; and

for applying feedback forces to the masses along y, these forces being intended to counterbalance the forces created by the Coriolis coupling along y.

These means generally consist of sets of electrodes. The x and y resonators therefore have various types of electrodes:

excitation electrodes 1, for applying an excitation force along x proportional to a control voltage U_(ex,x), and detection electrodes 2 that deliver a detection voltage U_(det,x) proportional to the movement along x;

detection electrodes 3 that deliver a detection voltage U_(det,y) proportional to the movement along y; and

feedback electrodes 4 which are in fact excitation electrodes for applying a feedback force to the y resonator proportional to a control voltage U_(ex,y).

The means 2 for detecting the movement of the mass along x are connected to the means 1 for applying excitation forces along the x direction via an oscillator 5 and an amplitude regulation device 6 placed in parallel with the oscillator 5.

An excitation or feedback loop for excitation along y comprises the following elements. The means 3 for detecting the movement of the mass along y are connected to the means 4 for applying feedback forces along the y direction by a shaping device 7, in series with a synchronous demodulator 8, a corrector 9 and then a modulator 10. The output signal from the gyroscope comes from the corrector 9.

The object of the invention is to provide permanent feedback control of F_(y), for example by controlling the stiffness K_(y), so as to make the natural frequencies F_(y) and F_(x) equal. To do this, a feedback control loop is proposed, which includes F_(y)-modifying means 11 (shown in FIGS. 7 and 8) such as, for example, electrodes for controlling the stiffness K_(y), which are controlled on the basis of an error signal representative of the difference between F_(x) and F_(y). The error signal is determined as follows.

FIG. 3 shows schematically a curve representative of the variation of the amplitude (in dB) of the signal |U_(det,y)| coming from the electrodes for detecting the movement of the mass along y, as a function of the frequency in Hz of the excitation signal U_(ex,y) applied to the excitation electrodes. This curve shows a maximum when F_(x)=F_(y) and decreases otherwise.

By disturbing the frequency of the excitation signal U_(ex,y), that is to say by applying a disturbing force along O_(y) to the mass, a disturbance of the detection signal, corresponding to the movement of the mass along y, is obtained, this disturbance being representative of the error signal.

The disturbing force is generated by applying, to the y excitation electrode 4, a disturbing voltage U_(ex,y) frequency-modulated about the central frequency F_(x) at the frequency F₀ of the following form: U _(ex,y) =U _(ex,0) sin(2π(F _(x) +ΔF sin(2πF ₀ t)t), U_(ex,0) being a constant.

U_(ex,y) is shown in FIG. 4 b) and obtained by applying, to an oscillator, a signal (in this case a voltage) for controlling the frequency modulation shown in FIG. 4 a).

FIG. 4 b) indicates certain frequencies of U_(ex,y).

In practice, the frequency modulation is not necessarily sinusoidal, but triangular. F₀ is chosen to be above the bandwidth of the gyroscope, but very much below F_(x). For example, ΔF is about 10% of F_(x).

Depending on whether the resonant frequency F_(y) is below, equal to or above the excitation frequency F_(x), the variations in the amplitude of the detection signal |U_(det,y)| will be different:

-   -   if F_(y)>F_(x), Δ|U_(det,y)|=u sin(2πF₀t) (sector 1, shown in         FIG. 5 a)     -   if F_(y)=F_(x), Δ|U_(det,y)|=u sin(4πF₀t) (sector 2, shown in         FIG. 5 b)     -   if F_(y)<F_(x), Δ|U_(det,y)|=−u sin(2πF₀t) (sector 3, shown in         FIG. 5 c).

These variations in the amplitude of the detection signal |U_(det,y)| are thus representative of the difference in F_(x) and F_(y): the error signal e is deduced from this difference.

Depending on the sector in question, the amplitude of the error signal is a signal of frequency F₀ in phase with the control signal (sector 1) or in phase opposition (sector 3) or a signal of frequency 2F₀ (sector 2).

These three situations are illustrated in FIGS. 5 a), 5 b) and 5 c), respectively. Each case shows the same curve as that in FIG. 3 and the variation in the signal for controlling the frequency modulation of U_(ex,y) as shown in FIG. 4 a), and the corresponding variation in the amplitude of the detection signal Δ|U_(det,y)| from which the error signal e is deduced.

In the case of FIG. 5 a) where F_(x)<F_(y), Δ|U_(det,y)| is a signal of frequency F₀ in phase with the control signal.

In the case of FIG. 5 b) where F_(x)=F_(y), Δ|U_(det,y)| is a signal of frequency 2F₀.

In the case of FIG. 5 c) where F_(x)>F_(y), Δ|U_(det,y)| is a signal of frequency F₀ in phase opposition with the control signal.

FIG. 6 a) shows the detection signal U_(det,y), the envelope of which is shown as Δ|U_(det,y)| in the case of which F_(x)≠F_(y). A demodulation reference signal of frequency F₀ and the error signal e coming from the synchronous demodulation device 15 are shown in FIGS. 6 b) and 6 c) respectively.

A gyroscope according to the invention will now be described. It comprises, as shown in FIG. 7, in addition to the elements described in relation to FIG. 2 and identified by the same references, a signal generator 12 for generating a signal that disturbs the vibration of the mass along y, connected to the mass M, and a feedback control loop for slaving the resonant frequency F_(y) to the frequency F_(x).

The disturbing force is generated by applying, to the y excitation electrode 4, by means of the generator 12 such as a VCO (voltage-controlled oscillator) connected to the y excitation loop, a disturbing voltage U_(ex,y) frequency-modulated about the central frequency F_(x) at the frequency F₀. The control signal from the oscillator is that shown in FIG. 4 a).

The feedback control loop comprises the following elements.

The amplitude of the signal U_(det,y) is recovered by means of an amplitude detector 13 after a shaping device 7 has shaped the signal coming from the detection electrodes 3. This detector 13 delivers |U_(det,y)| and, after the signal |U_(det,y)| has passed through an F₀-centered narrow band-pass filter 14 and then through an F₀ reference frequency demodulator 15, an error signal e is produced, which becomes zero when the frequency F_(y) becomes equal to F_(x).

After integration by means of an integrator/corrector 16, this error signal may control a voltage V on the stiffness electrode 11 that modifies the stiffness K_(y) and therefore the frequency F_(y).

The natural frequency F_(y) of the mass M along y is therefore properly slaved to the natural frequency F_(x) along x.

In the case described above, a disturbing force was applied to the mass along y by modulating the frequency of the excitation signal.

Rather than modulating the excitation frequency, it is possible, according to a variant of the invention, to modulate the amplitude of the electrostatic stiffness.

In this case, a voltage V+v₀ sin(2πF₀t) is applied to the stiffness electrode 11. The effect on the detection signal is then equivalent to that obtained by modulating the frequency of the excitation signal.

FIG. 8 shows the gyroscope corresponding to this variant. The disturbing force is then generated by applying, to the y stiffness electrode 11, the disturbing voltage v₀ sin(2πF₀t) generated by an oscillator (12′) centered on the frequency F₀, connected to the feedback control loop for slaving F_(y) to F_(x). The feedback control loop is the same as that described in relation to FIG. 7.

The various elements described in relation to FIGS. 2, 7 and 8 may of course be produced in analogue or digital technology.

The vibrating gyroscope according to the invention may have a plane or three-dimensional structure. It may or may not be micromachined. 

1. A gyroscope comprising at least one mass capable of vibrating along an x axis at a resonant excitation frequency F_(x) and capable of vibrating along a y axis perpendicular to the x axis, at a resonant detection frequency F_(y), under the effect of the Coriolis force generated by a rotation about a z axis perpendicular to the x and y axes, comprising connected to the mass, a signal generator for generating a signal that disturbs the vibration of the mass along y, and a feedback control loop for controlling the resonant frequency F_(y) so that F_(y) is equal or practically equal to F_(x) throughout the duration of use of the gyroscope, the feedback control loop comprising: means for modifying the resonant detection frequency F_(y); means for detecting the variation induced by the disturbing signal on the vibration of the mass along y, an error signal e representative of the difference between F_(x) and F_(y) being deduced from this variation; and control means for controlling the F_(y)-modifying means, the control being established on the basis of the error signal e.
 2. The gyroscope as claimed in claim 1, wherein the disturbing-signal generator is connected to the mass via the F_(y)-modifying means.
 3. The gyroscope as claimed in claim 1, wherein the disturbing-signal generator is connected to the F_(y)-modifying means via the feedback control loop.
 4. The gyroscope as claimed in claim 2, wherein the disturbing-signal generator is an oscillator of predetermined reference frequency F₀.
 5. The gyroscope as claimed in claim 2, wherein, since the gyroscope has a predetermined bandwidth, the disturbing signal is a periodic signal of frequency F₀, where F₀ is above the bandwidth of the gyroscope but below F_(x).
 6. The gyroscope as claimed in claim 1, comprising: excitation means for exciting the mass along y, with the aim of counterbalancing the vibration along y generated by the Coriolis force, wherein the disturbing-signal generator is connected to the mass via these excitation means.
 7. The gyroscope as claimed in claim 1, comprising: a y excitation loop and wherein the disturbing-signal generator is connected to the excitation means via the y excitation loop.
 8. The gyroscope as claimed in claim 6, wherein the disturbing-signal generator is a voltage-controlled oscillator.
 9. The gyroscope as claimed in claim 6, wherein, since the gyroscope has a predetermined bandwidth, the disturbing signal is a periodic signal, the frequency of which varies between F_(x)−ΔF and F_(x)+ΔF according to a frequency F₀, where F₀ is above the bandwidth of the gyroscope but below F_(x), ΔF being equal to about 10% of F_(x).
 10. The gyroscope as claimed in of claim 6, wherein the excitation means comprise electrodes.
 11. The gyroscope as claimed in claim 1, wherein the feedback control loop further comprises: connected in series, means for shaping the signal output by the detection means, an amplitude detection device, an F₀-centered band-pass filter, a synchronous demodulator for synchronizing with the reference frequency F₀, and an integrator/corrector that is connected to the means for modifying the frequency F_(y).
 12. The gyroscope as claimed in claim 1, wherein, since the mass is connected to a rigid frame by means of springs along x and y, of respective stiffness K_(x) and K_(y), the means for modifying the resonant frequency F_(y) comprise electrodes for controlling the stiffness K_(y).
 13. The gyroscope as claimed in claim 1, wherein the means for detecting the variation induced in the vibration of the mass along y comprise electrodes.
 14. The gyroscope as claimed in claim 1, wherein, when the disturbing signal is a periodic signal of predetermined frequency F₀, the disturbing signal is a sinusoidal or triangular signal.
 15. The gyroscope as claimed in claim 1, wherein the gyroscope is a micromachined gyroscope having a plane structure and in that the x and y axes lie in the plane of the plane structure.
 16. The gyroscope as claimed in claim 1, wherein the gyroscope is a micromachined gyroscope having a plane structure and in that the x axis lies in the plane of the plane structure and the y axis does not lie in the plane of the plane structure.
 17. The gyroscope as claimed in claim 1, wherein the gyroscope has a three-dimensional structure. 